I wouldnt trust what Drs say just because they say it. They do have a track record of making mistakes! 500mg one off doses of DMSA , no wonder you are getting to feel unwell.

When I have taken DMSA its 12.5 mg 4 hourly for three days. Even that can feel a bit too much and It has been two and a half years post removal for me. I would also question your Drs use of EDTA I know. I know Andy Cutler does not believe it to be effective for mercury chelation. Good luck